linux-kernel - Re: [PATCH -mm 4/5][AIO] - AIO completion signal notification

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Date:	Wed, 29 Nov 2006 14:08:01 +0100
From:	Sébastien Dugué <sebastien.dugue@...l.net>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-aio <linux-aio@...ck.org>, Andrew Morton <akpm@...l.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	Zach Brown <zach.brown@...cle.com>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Ulrich Drepper <drepper@...hat.com>,
	Jean Pierre Dion <jean-pierre.dion@...l.net>
Subject: Re: [PATCH -mm 4/5][AIO] - AIO completion signal notification

On Wed, 29 Nov 2006 10:51:50 +0000, Christoph Hellwig <hch@...radead.org> wrote:

> I'm a little bit unhappy about the usage of the notify flag.  The usage
> seems correct but very confusing:

  Well, I followed the logic from posix-timers.c, but it may be a poor
choice ;-)

  For a start, the SIGEV_* flags are quite confusing (for me at least).
SIGEV_SIGNAL is defined as 0, SIGEV_NONE as 1 and SIGEV_THREAD_ID as 4. I
would rather have seen SIGEV_NONE defined as 0 to avoid all this.

  I also wish I knew why those SIGEV_* constants were defined that way.

> 
> In io_submit_one we set ki_notify.notify to SIGEV_NONE and possibly
> call aio_setup_sigevent:
> 
> > +	/* handle setting up the sigevent for POSIX AIO signals */
> > +	req->ki_notify.notify = SIGEV_NONE;
> > +
> > +	if (iocb->aio_sigeventp) {
> > +		ret = aio_setup_sigevent(&req->ki_notify,
> > +					 (struct sigevent __user *)(unsigned
> > long)
> > +					 iocb->aio_sigeventp);
> > +		if (ret)
> > +			goto out_put_req;
> > +	}
> > +
> 
> aio_setup_sigevent then checks the user passed even for which notify type
> we have, and returns if it's none or otherwise sets notify->notify to it.
> 
> > +	if (event.sigev_notify == SIGEV_NONE)
> > +		return 0;
> > +
> > +	notify->notify = event.sigev_notify;
> 
> Later aio_setup_sigevent gets a reference to the target task_structure
> if notify->notify is (SIGEV_SIGNAL|SIGEV_THREAD_ID) but _always_ stores
> the target pointer.

  Yep, as SIGEV_SIGNAL is 0, this in fact checks that notify is SIGEV_THREAD_ID.
It could have been written as:

	if (notify->notify == SIGEV_THREAD_ID)

  And the target pointer is always store because at this point notify is either
SIGEV_SIGNAL or SIGEV_THREAD_ID.

> 
> > +	if (notify->notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID)) {
> > +		/*
> > +		 * This reference will be dropped in really_put_req() when
> > +		 * we're done with the request.
> > +		 */
> > +		get_task_struct(target);
> > +	}
> > +
> > +	notify->target = target;
> 
> 
> Once we're done with the iocb aio_complete aclls aio_send_signal if
> notify.notify is not SIGEV_NONE.

  Again, if it's not SIGEV_NONE, then it's either SIGEV_SIGNAL or
SIGEV_THREAD_ID.

> 
> > +	if (iocb->ki_notify.notify != SIGEV_NONE) {
> > +		ret = aio_send_signal(&iocb->ki_notify);
> > +
> > +		/* If signal generation failed, release the sigqueue */
> > +		if (ret)
> > +			sigqueue_free(iocb->ki_notify.sigq);
> > +	}
> > +
> 
> Which then uses notify->target to send the signal:
> > +	if (notify->notify & SIGEV_THREAD_ID)
> > +		ret = send_sigqueue(notify->signo, sigq, notify->target);
> > +	else
> > +		ret = send_group_sigqueue(notify->signo, sigq, notify->target);
> 
> And finally really_put_req puts the target if notify.notify contains
> either SIGEV_SIGNAL or SIGEV_THREAD_ID.
> 
> > +	/* Release task ref */
> > +	if (req->ki_notify.notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID))
> > +		put_task_struct(req->ki_notify.target);

  Could have been if (req->ki_notify.notify == SIGEV_THREAD_ID)

> 
> Do you see the confusing?  I think all the notify.notify != SIGEV_NONE
> in the above code should be replaces by the much more descriptive
> notify.notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID). In addition we should
> only store the target pointer inside the (SIGEV_SIGNAL|SIGEV_THREAD_ID)
> if block that gets a reference to it.

  No, I don't think so, notify == (SIGEV_SIGNAL|SIGEV_THREAD_ID) really means
notify == SIGEV_THREAD_ID which leaves out notify == SIGEV_SIGNAL. Or
am I grossly mistaken?

  Thanks,

  Sébastien.



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